There are situations in which it is necessary to inject or feed one liquid into the stream of another liquid. Some liquid pumping systems require an occasional injection of liquid while others need a more continuous feed of the liquid. Still others might require a combination of the two. For purposes of this disclosure, it is understood that the term “feed” will include inject.
One such common application is in the field of water treatment wherein certain chemicals, such as chlorinating solutions, fluorination chemicals and other liquids, are fed into the water stream at a point prior to its delivery for end use by consumers. It is important to maintain certain percentage levels of these added liquids in order to assure adequate functionality without exceeding predetermined concentrations which could be objectionable or even harmful to the consumer.
A variety of apparatus is available in the industry to perform this chemical feed task. Such apparatus typically takes the form of a pump, wherein pump speed and chemical feed rate is controlled by well known electronic means which employs chemical concentration detection means and provides voltage or current signal output for use by the pump drive system to adjust its feed rate. This system operates in a closed loop fashion to maintain a relatively stable concentration of the desired chemical in the water stream.
Certain chemicals, particularly sodium hypochlorite (NaOCl) solution used for chlorination of the water system, exhibit the troublesome characteristic of constant gas generation. Specifically, the liquid NaOCl spontaneously outgases in such a way that bubbles form in conduit piping, fittings and any other cavities in the feed circuit. Positive displacement pumps attempting to draw this liquid from storage tanks and feed it into the water stream can become gas-bound when encountering such gas bubbles. Once gas-bound, the pump will simply work against a “springy” bubble, which will alternately compress and expand to entirely devour the pump's displacement stroke volume. At this point, feeding of liquid chemical into the water stream ceases and the pump will uselessly run without effect.
This problem is aggravated by the often encountered requirement to feed the liquid chemical directly into a pressurized water stream. Here, even a modest sized gas bubble will give rise to a gas bound condition as the pump unsuccessfully attempts to compress the gas sufficiently to force it out of the pump chamber against the water stream back pressure. The problem is sufficiently severe that certain water treatment facilities undertake the extra step of diluting the sodium hypochlorite solution in the liquid chemical supply tank in order to reduce gas bubble formation. It can be reliably stated that the most aggravating problem known in the water chlorination and disinfection industry is the off-gas generated by the sodium hypochlorite NaOCl solution.
Another related problem is associated with priming. Once a chemical vessel is emptied, the feed apparatus will draw in air and entirely fill the intake circuit (including tubing, fittings, internal chambers and such) with this air. The chemical concentration detection apparatus will then signal or alarm for intervention by a technician. Chemical feed restoration now requires that a full liquid chemical vessel be substituted for the empty vessel followed by a troublesome and time consuming sequence of valve openings/closings by a skilled technician to bleed offending air out of the circuit in order to prime the pump. Only after the technician confirms by observation that the feed pump is actually feeding liquid into the water stream can the task be considered completed. This problem of manual bleeding is common to any liquid chemical application and is in addition to and apart from the out-gassing characteristics of NaOCl solutions.
Numerous attempts have been made to solve the problems described herein. For example, it is known in the field to incorporate a solenoid operated purge valve in a liquid pump, which is manually or automatically operated to divert the pressure output port of the feed pump away from the pressurized water stream and back to the liquid chemical supply tank. Once liquid has filled the pump circuit, the valve is shifted back so as to direct the chemical liquid into the pressurized water stream. However, the drawbacks of such prior art solutions include complex electronics, additional valves, manual intervention or urgent attention on the part of technicians.
Accordingly, it is desirable to provide a simply designed system, wherein gas bubbles are dispatched automatically while replacement of an empty liquid chemical supply tank and commissioning of a new full tank is simply done by switching input tubing from the empty to the full tank. It would be further desirable to provide an apparatus requiring no priming and does not require the pump to be turned off when changing liquid supplies. It would also be desirable for such an apparatus to be substantially leak-free and less prone to chemical precipitate build-up with resultant mechanical failure.